8137( 4) - 04/22/99 16:53 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8137 Version: 4 Check-in Date: 22-Apr-1999 16:52:36 1.Proposal Title: Intermediate-Age Dwarf Spheroidal Galaxies in the M81 Group ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 8 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Mario Mateo University of Michigan CoI: Ms. Julia Plummer University of Michigan N CoI: Dr. Carme Gallart Carnegie Observatories N CoI: Prof. Antonio Aparicio Instituto de Astrofi sica de Canarias N CoI: Dr. Robbie Dohm-Palmer University of Michigan N ------------------------------------------------------------------------------------ 5. Abstract A major goal of stellar populations studies of galaxies is to reconstruct the star- formation history (SFH) of the Local Universe via observations of the SFHs of individual, nearby galaxies and compare the result with the SFH history derived from high-redshift galaxies. This task is nearly complete in the Local Group (LG), but the agreement with high redshift observations is poor. Perhaps more local galaxies must be measured to achieve a representative sample. We propose to help enlarge the sample of nearby galaxies with good SFH determinations by obtaining deep V, I WFPC2 observations of UGC 4998 and DDO 44, both early-type dwarf members of the M 81 group that exhibit clear signs of young and intermediate-age populations. These galaxies are complementary to the mostly old, low-surface brightness M81 dwarfs observed to date with HST. Our observations will also allow us to (a) identify radial population gradients as a function of time in both galaxies, (b) determine their ba sic chemical properties, (c) critically compare distances derived using surface- brightness fluctuations in ground-based data, with distances derived from the HST data using the tip of the red giant branch, (d) place the galaxies in the global relations for other early-type systems that relate metallicity, size, surface brightness and luminosity, and (e) in UGC 4998, compare the stellar populations of its nucleus and off-center globular cluster. ------------------------------------------------------------------------------------ 8137( 4) - 04/22/99 16:53 - [ 2] Observations Description ------------------------ Exposure Time Justification: Our science requires good photometry of red giants in the target galaxies. We have assumed D = 3.6 Mpc, and E(B-V) = 0.03 mag throughout (Caldwell et al. 1998). Using the WFPC2 exposure time calculator, we estimate that 9 * 1600 = 14400 sec exposures in I (F814W) and 9 * 1700 = 15300 sec exposures in V (F555W) and a 3x3 dither pattern will allow us to achieve S/N = 9 at M_V = M_I = 0.0, or V = I = 27.7. These exposures can be fit into six CVZ orbits, meaning that we require 12 total orbits to observe both targets. Without the CVZ, we would require 13 orbits per galaxy to achieve the same S/N limit. Crowding Considerations: The most crowded field region in our exposures will be the core of UGC 4998. Assuming an old or intermediate-age population in that region, we estimate that the stellar density will correspond to approximately 30, 000 stars above the level of the lower subgiant branch within the PC field, with a maximum density of one such s tar for each 8-10 pixels. The latter corresponds to a typical separation of about 3 pixels in the core. This is sufficiently crowded that we will have to determine corrections to our photometry and star counts based on false-star experiments. Photometry of individual stars in the nucleus and globular cluster of UGC 4998 will clearly be significantly affected by crowding. We will carry out extensive false-star experiments to be certain that any differences observed in the stellar populations of these two environments are carefully validated or identified as crowding artifacts. Reduction Issues: We shall employ techniques developed extensively for other HST observations to constrain the SFHs of our target galaxies (Dohm-Palmer et al. 1997a, b; Gallart et al. 1998). To summarize, these methods involve counts of stars in well-defined evolutionary phases within the observed color-magnitude diagrams. For young populations, the luminosities of He-burning blue-loop stars are d ir ectly related to their age. For older populations, the luminosit ies of red supergiants can constrain ages, while for true intermediate-age or old stars, extensive models to reproduce the observed red-giant branch and AGB are necessary. In these cases, a best model is identified via least-squares iterative `fitting' by comparing the data to models constructed with a wide range of plausible SFHs, chemical enrichment histories, IMFs, binary fractions, reddenings, and completeness corrections as a function of color and magnitude. We have all the tools required to carry these analyses out, as well as the tools and expertise to obtain the highest- quality photometry from the proposed observations. Real Time Justification ----------------------- We request CVZ time for these observations to probe probe the SFHs of UGC 4998 and DDO 44 as far back in time as practical. This demands deep photometry -- our specific goal is S/N = 9 at V = I = 27.7. The resulting high-quality photometry (S/N > 15) of the red-giant branches in UGC 4998 and DDO 44 will allow us to determine metallicities and distances precisely for each system (Da Costa and Armandroff 1990; Lee et al. 1993; Aparicio 1994). The deep observations possible with the CVZ option also provide an opportunity to detect main- sequence turnoff stars to ages approaching 1 Gyr (Dohm-Palmer et al. 1997a, b). Both targets are deep in the Northern CVZ and both can be observed fully within a single SAA window. Because of the high overheads involved for multiple observations during normal orbits, we would require roughly 2.1-2.2 times as many orbits to achieve the same photometric S/N limits using non-CVZ time. The proximity of most M81 dwarfs near the CVZ makes this group p articularly important as a long-term target to determine SFHs and metallicities of galaxies beyond the Local Group. For UGC 4998 we require that as much of the central annular region containing the bluest stars is in the PC, along with the nucleus itself, and the galaxy's globular cluster (Figure 1). As noted above, this region has the highest stellar density in our targets and so must take advantage of the improved resolution of the PC. This imposes some very slight constraints on the telescope roll angle. No roll constraints are necessary for DDO 44. MM and JP are completing a three-color survey of dwarfs in the M81 group and from which we have chosen our targets. We are also carrying out SBF measurements of these systems independent of the HST observations. We hope to use these results to identify and correct for stellar population effects that might systematically affect SBF distances. We are also obtaining global photometric parameters for the galaxies from these gr ound-based data. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 8137( 4) - 04/22/99 16:53 - [ 3] Data Distribution Paper Products: NO Media: DAT Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 8137( 4) - 04/22/99 16:53 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 UGC4998 GALAXY, AMORPHOUS RA=9H 25M 12.00S +/- 2.0", J2000 V = 16.0+/-0.2 IRREGULAR DEC=68D 23' 2.0" +/- 2.0" B-V = 0.55+/-0.05 SURF(V) = 23.5+/-0.5 Comments: To be centered on PC. 2 DDO44 GALAXY, DWARF RA=7H 34M 12.14S +/- 2", DEC=66D J2000 V = 17.0+/-0.5 SPHEROIDAL 52' 59.1" +/- 2" B-V = 0.7+/-0.1 SURF(V) = 25.0+/-0.5 Comments: Offset 5 arcsec towards PA=170 of true center; orient will avoid nearby bright star. 8137( 4) - 04/22/99 16:53 - [ 5] Visit: 01 Visit Priority: Visit Requirements: CVZ ORIENT 240D TO 280D On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 UGC4998 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO,CLOCKS=NO,READ=YES 1 1500 S ------------------------------------------------------------------------------------------------------------------------------------ 2 UGC4998 WFPC2 IMAGE PC1 F555W CR-SPLIT=NO,CLOCKS=NO,READ=YES 1 1600 S ------------------------------------------------------------------------------------------------------------------------------------ 3 UGC4998 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO,READ=YES,CLOCKS=NO 1 1500 S POS TARG 0.1180, 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 4 UGC4998 WFPC2 IMAGE PC1 F555W CLOCKS=NO,CR-SPLIT=NO,READ=YES 1 1600 S POS TARG 0.1180, 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 5 UGC4998 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO,CLOCKS=NO,READ=YES 1 1500 S POS TARG 0.2361, 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 6 UGC4998 WFPC2 IMAGE PC1 F555W READ=YES,CR-SPLIT=NO,CLOCKS=NO 1 1600 S POS TARG 0.2361, 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 7 UGC4998 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO,READ=YES,CLOCKS=NO 1 1500 S POS TARG 0.0590,-0.1180 ------------------------------------------------------------------------------------------------------------------------------------ 8 UGC4998 WFPC2 IMAGE PC1 F555W CLOCKS=NO,CR-SPLIT=NO,READ=YES 1 1600 S POS TARG 0.0590,-0.1180 ------------------------------------------------------------------------------------------------------------------------------------ 9 UGC4998 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO,READ=YES,CLOCKS=NO 1 1500 S POS TARG 0.1770,-0.1180 ------------------------------------------------------------------------------------------------------------------------------------ 10 UGC4998 WFPC2 IMAGE PC1 F555W CLOCKS=NO,CR-SPLIT=NO,READ=YES 1 1600 S POS TARG 0.1770,-0.1180 ------------------------------------------------------------------------------------------------------------------------------------ 11 UGC4998 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO,READ=YES,CLOCKS=NO 1 1500 S POS TARG 0.2950,-0.1180 ------------------------------------------------------------------------------------------------------------------------------------ 12 UGC4998 WFPC2 IMAGE PC1 F555W CLOCKS=NO,CR-SPLIT=NO,READ=YES 1 1600 S POS TARG 0.2950,-0.1180 ------------------------------------------------------------------------------------------------------------------------------------ 13 UGC4998 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO,READ=YES,CLOCKS=NO 1 1500 S POS TARG 0.1180,-0.2361 ------------------------------------------------------------------------------------------------------------------------------------ 14 UGC4998 WFPC2 IMAGE PC1 F555W CLOCKS=NO,CR-SPLIT=NO,READ=YES 1 1600 S POS TARG 0.1180,-0.2361 ------------------------------------------------------------------------------------------------------------------------------------ 15 UGC4998 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO,READ=YES,CLOCKS=NO 1 1500 S POS TARG 0.2361,-0.2361 ------------------------------------------------------------------------------------------------------------------------------------ 16 UGC4998 WFPC2 IMAGE PC1 F555W CLOCKS=NO,CR-SPLIT=NO,READ=YES 1 1600 S POS TARG 0.2361,-0.2361 ------------------------------------------------------------------------------------------------------------------------------------ 17 UGC4998 WFPC2 IMAGE PC1 F814W CR-SPLIT=NO,READ=YES,CLOCKS=NO 1 1400 S POS TARG 0.3541,-0.2361 ------------------------------------------------------------------------------------------------------------------------------------ 18 UGC4998 WFPC2 IMAGE PC1 F555W CLOCKS=NO,CR-SPLIT=NO,READ=YES 1 1600 S POS TARG 0.3541,-0.2361 ------------------------------------------------------------------------------------------------------------------------------------ 8137( 4) - 04/22/99 16:53 - [ 6] Visit: 02 Visit Priority: Visit Requirements: ORIENT 150D TO 190D CVZ On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 DDO44 WFPC2 IMAGE PC1 F814W CLOCKS=NO,READ=YES,CR-SPLIT=NO 1 1500 S ------------------------------------------------------------------------------------------------------------------------------------ 2 DDO44 WFPC2 IMAGE PC1 F555W CR-SPLIT=NO,CLOCKS=NO,READ=YES 1 1600 S ------------------------------------------------------------------------------------------------------------------------------------ 3 DDO44 WFPC2 IMAGE PC1 F814W CLOCKS=NO,CR-SPLIT=NO,READ=YES 1 1500 S POS TARG 0.1180, 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 4 DDO44 WFPC2 IMAGE PC1 F555W CR-SPLIT=NO,CLOCKS=NO,READ=YES 1 1600 S POS TARG 0.1180, 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 5 DDO44 WFPC2 IMAGE PC1 F814W CLOCKS=NO,CR-SPLIT=NO,READ=YES 1 1500 S POS TARG 0.2361, 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 6 DDO44 WFPC2 IMAGE PC1 F555W CR-SPLIT=NO,CLOCKS=NO,READ=YES 1 1600 S POS TARG 0.2361, 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 7 DDO44 WFPC2 IMAGE PC1 F814W CLOCKS=NO,CR-SPLIT=NO,READ=YES 1 1500 S POS TARG 0.0590,-0.1180 ------------------------------------------------------------------------------------------------------------------------------------ 8 DDO44 WFPC2 IMAGE PC1 F555W CR-SPLIT=NO,CLOCKS=NO,READ=YES 1 1600 S POS TARG 0.0590,-0.1180 ------------------------------------------------------------------------------------------------------------------------------------ 9 DDO44 WFPC2 IMAGE PC1 F814W CLOCKS=NO,CR-SPLIT=NO,READ=YES 1 1500 S POS TARG 0.1770,-0.1180 ------------------------------------------------------------------------------------------------------------------------------------ 10 DDO44 WFPC2 IMAGE PC1 F555W CR-SPLIT=NO,CLOCKS=NO,READ=YES 1 1600 S POS TARG 0.1770,-0.1180 ------------------------------------------------------------------------------------------------------------------------------------ 11 DDO44 WFPC2 IMAGE PC1 F814W CLOCKS=NO,CR-SPLIT=NO,READ=YES 1 1500 S POS TARG 0.2950,-0.1180 ------------------------------------------------------------------------------------------------------------------------------------ 12 DDO44 WFPC2 IMAGE PC1 F555W CR-SPLIT=NO,CLOCKS=NO,READ=YES 1 1600 S POS TARG 0.2950,-0.1180 ------------------------------------------------------------------------------------------------------------------------------------ 13 DDO44 WFPC2 IMAGE PC1 F814W CLOCKS=NO,CR-SPLIT=NO,READ=YES 1 1500 S POS TARG 0.1180,-0.2361 ------------------------------------------------------------------------------------------------------------------------------------ 14 DDO44 WFPC2 IMAGE PC1 F555W CR-SPLIT=NO,CLOCKS=NO,READ=YES 1 1600 S POS TARG 0.1180,-0.2361 ------------------------------------------------------------------------------------------------------------------------------------ 15 DDO44 WFPC2 IMAGE PC1 F814W CLOCKS=NO,CR-SPLIT=NO,READ=YES 1 1500 S POS TARG 0.2361,-0.2361 ------------------------------------------------------------------------------------------------------------------------------------ 16 DDO44 WFPC2 IMAGE PC1 F555W CR-SPLIT=NO,CLOCKS=NO,READ=YES 1 1600 S POS TARG 0.2361,-0.2361 ------------------------------------------------------------------------------------------------------------------------------------ 17 DDO44 WFPC2 IMAGE PC1 F814W CLOCKS=NO,CR-SPLIT=NO,READ=YES 1 1400 S POS TARG 0.3541,-0.2361 ------------------------------------------------------------------------------------------------------------------------------------ 18 DDO44 WFPC2 IMAGE PC1 F555W CR-SPLIT=NO,CLOCKS=NO,READ=YES 1 1600 S POS TARG 0.3541,-0.2361 ------------------------------------------------------------------------------------------------------------------------------------ 8137( 4) - 04/22/99 16:53 - [ 7] Summary Form for Proposal 8137 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures PC1 ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO CLOCKS=NO READ=YES ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements CVZ ORIENT 240D TO 280D POS TARG 0.1180, 0.0 POS TARG 0.2361, 0.0 POS TARG 0.0590,-0.1180 POS TARG 0.1770,-0.1180 POS TARG 0.2950,-0.1180 POS TARG 0.1180,-0.2361 POS TARG 0.2361,-0.2361 POS TARG 0.3541,-0.2361 ORIENT 150D TO 190D CVZ ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F814W F555W ------------------------------------------------------------------------------------------------------------------------------------ Target Names UGC4998 DDO44 ------------------------------------------------------------------------------------------------------------------------------------